Testing of chemicals

Adverse Outcome Pathways, Molecular Screening and Toxicogenomics


The OECD Environmental, Health and Safety (EHS) Programme has been helping member countries to make better use of increased knowledge of how chemicals induce adverse effects in humans and wildlife, through the so-called Adverse Outcome Pathways.

What's new

OECD releases seven publications on Adverse Outcome Pathways (AOPs)


All publications in the Series on Adverse Outcome Pathways can be found here

What is an Adverse Outcome Pathway?

The OECD launched a new programme on the development of Adverse Outcome Pathways (AOP) in 2012. An AOP is an analytical construct that describes a sequential chain of causally linked events at different levels of biological organisation that lead to an adverse health or ecotoxicological effect (see figure below). AOPs are the central element of a toxicological knowledge framework being built to support chemical risk assessment based on mechanistic reasoning.


Schematic representation of the AOP illustrated with reference to a number of pathways

AOP image


Watch our video: OECD work on Adverse Outcome Pathway


The AOP Development Programme

The AOP development programme is overseen by the extended Advisory Group on Molecular Screening and Toxicogenomics (EAGMST). EAGMST members are nominated by their National Coordinators; they play an active role in the development of AOPs, as well as in the internal review process.

Representation of the AOP Development Process

‌ AOP Description of the development process


AOP Knowledge Base and Tools

The OECD's AOP Knowledge Base tools, constantly developed and refined, are a web-based platform which aims to bring together all knowledge on how chemicals can induce adverse effects, therefore providing a focal point for AOP development and dissemination.

Logo e AOP Portal


  • The e AOP Portal is the main entry point of the AOP Knowledge Base. A search engine, the Portal enables search by key words in AOP titles and key events in the AOP Wiki and Effectopedia platforms. It houses the status of all AOPs in the OECD Work Plan and the official copy of OECD endorsed AOPs.

logo AOP Wiki

  • The AOP Wiki provides a system that organises, via crowd-sourcing, the available knowledge and published research into a verbal description of individual pathways, using a user friendly Wiki interface. Information on AOP is collected in a qualitative, narrative way.

Logo Effectopedia AOP

  • Effectopedia is an open-knowledge and structured platform able to display information on Adverse Outcome Pathways. It uses visually expressed modular structures to capture semantically annotated knowledge and algorithms together with all the supporting evidence.

Find out more

Test Guidelines Programme

The OECD Test Guidelines Programme for the identification of new biomaker endpoints and in vitro test methods that are candidates to become part of OECD Test Guidelines.

Hazard Assessment Activities

The OECD Hazard Assessment activities for the development of Integrated Approaches to Testing and Assessment (IATA, using AOPs as a framework.
The OECD QSAR project for the identification of new methods/profilers for grouping chemicals.



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